Cellular DNA is continuously assaulted by metabolic sources such as oxygen radicals and by environmental agents to generate damage that is cytotoxic and threatens genetic stability. These dire effects are counteracted by DNA repair systems. Base excision repair (BER) acts on both endogenous DNA damage and lesions from diverse environmental agents. BER proteins have been individually characterized, but the coordination among BER proteins and the modulation of their expression are poorly understood. In mammalian cells, the central enzyme of BER is the apurinic (AP) endonuclease Apel, which incises AP sites produced by DNA glycosylases or DNA-damaging agents and removes oxidative deoxyribose fragments that block 3' termini. These reactions produce active primers for DNA repair synthesis. We have shown that Apel interacts with other BER enzymes to engage them with their substrates and stimulate their activities: Myh glycosylase for oxidative mispairs upstream in BER, and DNA polymerase B downstream. We will characterize the protein interaction sites, the participation of other players such as p53, and the effects of interactions on the rate and specificity of BER. We will engineer interaction-defective derivatives of Ape 1 and test whether they or Ape 1-derived peptides interfere with efficient BER in vitro and in vivo. Apel expression is regulated during the cell cycle and in response to oxidative stress. We will define the regions of the APE1 promoter involved in induction by oxidative stress, and use an antisense approach to test the importance of Apel in inducible cellular resistance to DNA-damaging agents. We will study the consequences of endogenous AP sites in S. cerevisiae by addressing mutational specificity, processing by lesion "bypass" DNA polymerases, and the influence of replication polarity. These studies will provide a holistic picture of the dynamic and interactive processes of BER and mutagenesis by AP sites.